1. Field of the Invention
The present invention relates to a projection type image display apparatus that enlarges and projects an image formed by a reflection type image forming device or a transmission type image forming device.
2. Description of the Related Art
Projection optical systems that enlarge and project images formed by image forming devices in projection type image display apparatuses are roughly classified into refraction optical systems composed of refraction type optical elements such as lenses, and reflection optical systems composed of reflection type optical elements such as mirrors. In general, since reflection optical systems have no chromatic aberration, they have a feature such that more fine images can be obtained.
When dust and dirt adhere to image forming devices, their images are projected onto enlarged and projected images, thereby causing deterioration in image quality. U.S. Pat. No. 6,280,036 B1 and Japanese Patent Application Laid-Open No. 2003-337380 disclose constitutions that prevent dust and dirt from adhering to image forming devices in projection type image display apparatuses that adopt refraction optical systems as projection optical systems. U.S. Pat. No. 6,280,036 B1 discloses the projection type image display apparatus which is constituted so that an opening of an optical housing is sealed by a field lens and a trimming filter, a lens barrel of a projection optical system is mounted to the optical housing, and the inside of the optical housing which houses an image forming device is sealed. Japanese Patent Application Laid-Open No. 2003-337380 discloses the projection type image display apparatus which is constituted so that an entire optical system excluding a light source (including an image forming device) is arranged in a sealed space.
A structure that effectively prevents adhesion of dust and dirt to the image forming devices is not, however, proposed for the projection type image display apparatuses that adopt the reflection optical systems as the projection optical systems. This is because it is difficult that the reflection optical systems adopt the sealed structure. Concretely, since the reflection optical systems have high sensitivity, it is necessary to adjust positions and angles of the reflection type optical elements such as curved mirrors at the time of manufacturing. In order to enable this adjustment, since it is necessary to secure an access to the reflection type optical elements, the adoption of the sealed structure is difficult. Further, since an optical path in the reflection optical systems bend and extend among a plurality of reflection type optical elements, a space of the optical path is large. This makes the adoption of the sealed structure difficult.